1. Field of the Invention
The present invention relates to photoelectric conversion devices such as solar cells, photosensors, and so on, and a production method of such devices and, particularly to photoelectric conversion devices, such as the solar cells, photosensors, etc., comprising a semiconductor layer with an uneven structure (concave-convex structure) which functions as a photoelectric conversion layer, and a production method thereof.
2. Related Background Art
The global environment has been being deteriorated by emissions of carbon dioxide, nitrogen oxides, etc., which are earth-warming gases, due to combustion of petroleum for thermal power generation, combustion of gasoline in automobile engines, and so on. There is also worry about future exhaustion of crude oil and thus attention is now focusing on photovoltaic power generation.
Thin-film crystal silicon (Si) solar cells can be made inexpensively because their power generation layers are thin and an use amount of Si raw material is small. Since crystal Si is used for the power generation layer, it can be expected to implement higher conversion efficiency and less deterioration than the solar cells of amorphous Si and the like. Further, because the thin-film crystal Si solar cells can be bent to some extent, they can be used with being glued on curved portions such as bodies of automobiles, household electrical appliances, roof tiles, and so on.
For realizing the thin-film crystal Si solar cells, Japanese Patent Application Laid-Open No. 8-213645 discloses separation of a thin-film single-crystal Si layer by making use of epitaxial layers on a porous Si layer. FIG. 19 is a cross-sectional view showing a method for formation of a thin-film crystal Si solar cell, which is disclosed in Japanese Patent Application Laid-Open No. 8-213645. In FIG. 19, reference numeral 101 designates a non-porous Si substrate, 102 a porous Si layer, 103 a p+ Si layer, 104 a pxe2x88x92 Si layer, 105 an n+ Si layer, 106 a protective film, 109 and 111 an adhesive, and 110 and 112 a jig. In the solar cell production method of FIG. 19, the structure having the porous Si layer 102 in the surface region of the non-porous Si substrate 101 can be formed by anodization of an Si wafer. After that, the p+ Si layer 103 is epitaxially grown on the porous Si layer 102, and the pxe2x88x92 Si layer 104 and n+ Si layer 105 are further grown thereon. Then the protective layer 106 is formed thereon. Then the protective layer 106 and non-porous Si substrate 101 are provided with the adhesive 111 and 109, respectively, and are bonded to the jig 112 and 110, respectively. After that, tensile force is exerted on the jig 112 and 110 to separate the non-porous Si substrate 101 from the epitaxial Si layers 103, 104 and 105 at the porous Si layer 102. Then the solar cell is formed using the epitaxial Si layers 103, 104 and 105, and the non-porous Si substrate 101 is put again into the same steps as described above, thereby decreasing the cost.
For increasing the photoelectric conversion efficiency, there are corrugated solar cells in which the front and back surfaces of the semiconductor layers are provided with an uneven shape (concave-convex shape). FIG. 20 is a perspective view of a semiconductor substrate of a solar cell disclosed by Uematsu et al. (xe2x80x9cHigh-efficiency solar cellsxe2x80x9d workshop, Sapporo (1989), A6, p31). The corrugated solar cells can demonstrate a high photoelectric conversion efficiency even with the semiconductor layers of thin films, because optical path lengths of incident light are long. The corrugated substrate of Uematsu et al. is produced by a method of carrying out anisotropic etching with etching masks on the both surfaces of Si wafer. As a result, the corrugated substrate is obtained in the substrate thickness (the wafer thickness) of about several ten xcexcm.
Further, Japanese Patent Application Laid-Open No. 4-355970 discloses that the solar cell having the uneven shape on the surface is formed by use of single-crystal Si layers in such a way that the (100) Si wafer is subjected to anisotropic etching to form V-shaped grooves, an oxide film is selectively formed on the surface thereof, the single-crystal Si layers are formed on the surface thereof, and the single-crystal Si layers are separated from the Si wafer.
The corrugated substrate production method of Uematsu, et al. illustrated in FIG. 20 necessitates the formation of the etching masks on the both surfaces of the semiconductor substrate and thus has to repeatedly use the high-cost steps of photolithography, etc. In addition, a large amount of Si is removed by etching, and this point is also the cause of high cost. Therefore, the solar cells and photosensors produced by use of this corrugated substrate suffer the drawback of high production cost, though demonstrating the high photoelectric conversion efficiency.
An object of the present invention is, therefore, to provide a production method of photoelectric conversion device that necessitates a smaller number of use times of the high-cost steps of photolithography and the like, that can reduce consumption of Si, and that permits reclaiming of the substrate. Another object of the present invention is to provide a photoelectric conversion device that can be provided at low production cost.
The present invention provides a production method of a photoelectric conversion device, the production method comprising a step of forming an uneven shape (concave-convex shape) in a surface of a substrate, a step of providing a separation layer maintaining the uneven shape on the substrate, a step of forming a semiconductor film maintaining the uneven shape on the separation layer, and a step of separating the semiconductor film from the substrate at the separation layer, wherein the step of forming the uneven shape in the surface of the substrate is a step of forming the substrate having the uneven shape on the surface by anisotropic etching of the substrate with the separation layer remaining after the separation.
In the above method, it is preferable that the step of providing the separation layer be a step of forming a porous semiconductor layer as the separation layer on a semiconductor substrate by anodization of the semiconductor substrate.
The substrate having the uneven shape on the surface or a substrate having an even surface can be used as the substrate with the separation layer remaining after the separation.
It is also preferable that the step of forming the uneven shape on the surface of the substrate be a step of forming regions non-parallel to the principal plane of the substrate and regions parallel to the principal plane of the substrate in the surface of the substrate. For implementing this, it is preferable that anisotropic etching be carried out by using masks provided on portions of the substrate, whereby the portions covered with the masks on the surface of the substrate are formed as the regions parallel to the principal plane of the substrate.
Further, the present invention provides photoelectric conversion devices produced by the above methods; i.e., the photoelectric conversion device comprising a semiconductor film having the uneven shape in a light incident surface and a surface opposite thereto, and a back reflecting layer provided on the surface side opposite to the light incident surface of the semiconductor film; the photoelectric conversion device comprising a semiconductor film having a substantially even light incident surface and the uneven shape in a surface opposite thereto, and a back reflecting layer provided on the surface side opposite to the light incident surface of the semiconductor film; and the photoelectric conversion device wherein an electrode is formed on each of portions formed on the regions of the semiconductor film parallel to the principal plane of the substrate.